Exhaust gas purification system for a gasoline engine

ABSTRACT

Subject of the invention is an exhaust gas purification system for a gasoline engine, comprising in consecutive order the following devices:
         a first three-way-catalyst (TWC1), a gasoline particulate filter (GPF) and a second three-way-catalyst (TWC2),   wherein the oxygen storage capacity (OSC) of the GPF is greater than the OSC of the TWC2, wherein the OSC is determined in mg/l of the volume of the device.       

     The invention also relates to methods in which the system is used and uses of the system.

The invention relates to an exhaust gas purification system for agasoline engine, comprising in consecutive order a firstthree-way-catalyst, a gasoline particulate filter and a secondthree-way-catalyst. The invention also relates to methods and usesthereof.

STATE OF THE ART

Exhaust gas from gasoline engines comprises pollutants which have to beremoved before the exhaust gas is released into the environment. Themost relevant pollutants in this regard are nitrogen oxides (NO_(x)),hydrocarbons (HC or THC), carbon monoxide (CO) and particulate matter,especially soot. Other pollutants comprise sulfur oxides (SO_(x)) andvolatile organic compounds (VOC). Such gaseous pollutants are removedfrom the exhaust gas by catalyst system and devices located downstreamof the engine. Typically, such systems comprise a three-way catalyst(TWC), which is capable of removing nitric oxides, hydrocarbons andcarbon monoxide.

In recent years, there has been increasing attention on the removal ofparticulate matter, and especially soot, from exhaust gas from gasolineengines. For many years, soot particles were only removed from dieselengine exhaust gas. However, there is growing evidence that also finesoot particles from gasoline engines can impair health. Therefore, thereis a strong tendency to equip exhaust gas purification systems forgasoline engines with gasoline particulate filters (GPF). The GPF shouldefficiently reduce the particle mass (PM) and the particle number (PN)of the exhaust gas. In the European Union, the standard EURO 6 datedSeptember 2014 defines for the first time maximum levels for particulatematter of gasoline engines of passenger vehicles.

Presently, there are increasing demands by public authorities onmanufacturers of vehicles and gasoline engines to improve exhaust gaspurity. There is a worldwide trend towards lower legal thresholds foremission levels of such pollutants. Moreover, the European Union plansto introduce obligatory tests for vehicles under real conditions withportable emissions measurement systems under the designation RealDriving Emissions (RDE). Such demands exert high pressure onmanufacturers to provide exhaust gas purification systems which fulfillall legal standards. In this regard, it is a known problem that theamount and ratio of exhaust gas pollutants can vary significantlydepending on the operation conditions of the engine. Vehicle exhaust gaspurification systems should be efficient under conditions as differentas urban traffic, long-distance traffic, low or high velocity, cold orhot environment, and cautious or offensive driving.

It is another problem that a gasoline engine should have low fuelconsumption in order to keep carbon dioxide emissions low. Carbondioxide is considered to be the main cause of the greenhouse effect andglobal warming. Thus, attempts to increase exhaust gas purity should notnegatively affect the performance of the gasoline engine.

It is a general problem that when a catalyst device, such as a TWC, iscombined with a GPF in purifying exhaust gas from a gasoline engine, itis difficult to achieve an efficient depletion of all relevantpollutants whilst maintaining a high performance of the gasoline engine.The reason is that three different properties of the exhaust gaspurification system, which are at least in part antagonistic, have to bebrought into accordance with each other. Accordingly, the system shouldhave high catalytic activity, high filtration efficiency and lowpressure drop. High filtration efficiency is required for efficientremoval of particles. High catalytic activity is required for efficientdepletion of gaseous pollutants, such as HC, NO_(x) and CO. Low pressuredrop is required for maintaining a high engine performance. In contrast,an increasing pressure drop tends to reduce the efficiency of theengine, which leads to higher fuel consumption and carbon dioxideemission.

It is a general problem that a GPF and catalytic devices require hightemperatures for efficient operation. Therefore, they have to beoperated close to the engine, which leads to dimensional limitations ofthe overall exhaust gas purification system. In order to increase thefiltration efficiency or catalytic activity of an exhaust gaspurification system, more catalyst material and filtration means arerequired. In the limited space of the catalytic system, this isgenerally associated with an increase of pressure drop in the GPF and incatalytic devices. The reason is that the exhaust gas is attenuated whenpassing filtering material and porous coatings in such devices.Consequently, the efficiency of the gasoline engine has to be reducedand more carbon dioxide is emitted for achieving a comparableperformance. On the other hand, if the pressure drop is kept low, it isdifficult to achieve a good filtration efficiency and high catalyticactivity in the limited space of the exhaust gas purification system.

Various attempts have been made in the prior art to provide gasolineengine purification systems, which overcome the above-mentionedproblems. For example, various catalytic devices and gasolineparticulate filters have been described, which are equipped withspecific catalysts or combination of catalysts. In this regard,catalysts can be provided with special combinations of metals orcatalyst layers. Other solutions focus on the internal structure orspecific physical properties of such devices.

Further, exhaust gas purification systems with special arrangements ofmultiple devices have been proposed for increased efficiency. Forexample, DE 10 2015 212 514 A1 suggests a combination of two consecutiveTWC and a GPF located downstream of the second TWC. WO 2010/052055 A1also discloses a combination of a first TWC, a second downstream TWC anda GPF further downstream. It is a problem of such systems that when theterminal GPF is regenerated and the stored soot is burned, newpollutants such as CO and HC can be formed and emitted into theenvironment. It is another problem that the terminal GPF is locatedrelatively far from the engine. Therefore, it cannot be heated rapidlyand efficiently by the engine exhaust gas to achieve the optimal processtemperature. Moreover, it is difficult to achieve the high regenerationtemperature required for efficient soot burning, so an activeregeneration has to be triggered. Generally, operation of a GPF at alow, non-optimal temperature reduces the efficiency and increases theamounts of residual pollutants. Therefore, the performance of suchsystems could still be improved.

WO 2017/004414 A1 discloses various systems for purifying exhaust gasfrom gasoline engines comprising N₂O removal catalysts. For example, anupstream TWC is coupled with downstream devices, such as a GPF and theN₂O removal catalyst. However, the terminal catalytic device is for N₂Oremoval and cannot efficiently remove pollutants formed in the upstreamGPF, such as CO and CH. The data provided in the document also indicatesthat the overall performance could still be improved.

WO 2010/096641 A1 also discloses a combination of an upstream TWC whichis close-coupled to a gasoline engine, a downstream particulate mattercontrol device and a downstream NO_(x) control system. However, theterminal catalytic device focusses on NO_(x) removal and cannotefficiently remove the most relevant pollutants from the upstream GPF,for example those which are formed during regeneration of the GPF.Overall, the performance of the system could still be improved.

Overall, there is a continuous need for providing exhaust gaspurification systems for gasoline engines, which overcome theabove-mentioned problems.

Problem Underlying the Invention

It is a problem underlying the invention to provide an exhaust gaspurification system for gasoline engines which overcome theabove-mentioned problems.

Specifically, an exhaust gas purification system for gasoline enginesshall be provided, which efficiently removes the relevant pollutants,and especially NO_(x), hydrocarbons (HC), carbon monoxide andparticulate matter, especially soot. The filtration efficiency for sootshould be high with regard to particle mass as well as particle number.At the same time, the pressure drop of the system should be low, suchthat the engine can maintain a high performance and carbon dioxideemissions will not increase.

It is a special problem to provide an exhaust gas purification systemfor gasoline engines, which is efficient under various differentoperation conditions. Therefore, the level of pollutants shall be lowalso under RDE conditions.

Further, routine monitoring of the system by on board diagnosis (OBDsystem) should be convenient and provide appropriate results.

It is a further problem underlying the invention to provide a systemwhich has high purification efficiency, but is relatively simple andusable in standard automobile applications. The system should berelatively compact, stable and convenient to manufacture and use.

DISCLOSURE OF THE INVENTION

Surprisingly, it was found that the problem underlying the invention issolved by an exhaust gas purification system according to the claims.Further embodiments of the invention are outlined throughout thedescription.

Subject of the invention is an exhaust gas purification system for agasoline engine, comprising in consecutive order the following devices:

-   -   a first three-way-catalyst (TWC1), a gasoline particulate filter        (GPF) and a second three-way-catalyst (TWC2),

wherein the oxygen storage capacity (OSC) of the GPF is greater than theOSC of the TWC2, wherein the OSC is determined in mg/l of the volume ofthe device.

The invention relates to an exhaust gas purification system for agasoline engine. A gasoline engine is a combustion engine, which usespetrol (gasoline) as fuel. A gasoline engine is different from a dieselengine, which does not use spark ignition. Generally, exhaust gasemitted from gasoline engines has a different composition than exhaustgas from diesel engines and requires different exhaust gas purificationsystems.

Preferably, the engine uses gasoline direct injection (GDI), also knownas petrol direct injection, because these engines are known for theirimproved fuel efficiency. Typically, the exhaust gas from such an enginecomprises a relatively high number of relatively small soot particles.Especially for such an engine, it can be advantageous that the system iscapable of efficient removal of soot particles.

The purification system comprises the three devices as outlined above.Typically, the devices are different units, which can be provided inseparate housings. The devices can be connected by connection means,such as tubes and/or plugs. The three devices are arranged inconsecutive order, such that the TWC1 is located upstream from the GPF,which is located upstream from the TWC2. The TWC1 is positioneddownstream from the gasoline engine. As used herein, the terms“upstream” and “downstream” refer to the directions of the flow of theengine exhaust gas stream from the engine towards the exhaust pipe, withthe engine in an upstream location and the exhaust pipe downstream.

The exhaust gas purification system comprises at least the threepurification devices TWC1, GPF and TWC2. In a preferred embodiment, thesystem does not comprise other purification devices, especially notadditional catalytic devices. More preferably, the system does notcomprise another TWC, another GPF and/or another pollutant removaldevice, such as a separate NO_(x) removal device or the like. Accordingto the invention, it was found that efficient exhaust gas purificationis possible only with the three devices in the order described herein.

In another embodiment, the system comprises additional devices whichparticipate in pollutant removal. In one embodiment, at least oneadditional catalyst device may be present. In another embodiment, atleast one additional non-catalytic device may be present.

A TWC comprises a three way catalyst coating which is coated on aflow-through substrate. The term “three-way” refers to the function ofthree-way conversion, where hydrocarbons, carbon monoxide, and nitrogenoxides are substantially simultaneously converted. Three-way-catalysts(TWC) are known and widely used in the art. A gasoline engine typicallyoperates under near stoichiometric reaction conditions that oscillate orare perturbated slightly between fuel-rich and fuel-lean air to fuelratios (A/F ratios) (λ=1+/−˜0.01), at perturbation frequencies of 0.5 Hzto 2 Hz. This mode of operation is also referred to as “perturbatedstoichiometric” reaction conditions. TWC catalysts include oxygenstorage materials (OSM) such as ceria that have multi-valent stateswhich allows oxygen to be held and released under varying air to fuelratios. Under rich conditions, when NO_(x) is being reduced, the oxygenstorage capacity (OSC) provides a small amount of oxygen to consumeunreacted CO and HC. Likewise, under lean conditions when CO and HC arebeing oxidized, the OSM reacts with excess oxygen and/or NO_(x). As aresult, even in the presence of an atmosphere that oscillates betweenfuel-rich and fuel-lean air to fuel ratios, there is conversion of HC,CO, and NO_(x) all at the same (or at essentially all the same) time.Typically, a TWC catalyst comprises one or more platinum group metalssuch as palladium and/or rhodium and optionally platinum; an oxygenstorage component; and optionally promoters and/or stabilizers. Underrich conditions, TWC catalysts may generate ammonia.

The term “platinum group metal” refers to the six platinum-group metalsruthenium, rhodium, palladium, osmium, iridium, and platinum.

A “gasoline particulate filter” (GPF) is a device for removingparticulate matter, especially soot, from exhaust gas. The GPF is awall-flow filter. In such a device, the exhaust gas passes the filterwalls inside the device, whereas the particles are not capable ofpassing the filter walls and accumulate inside the device. Typically,the filters comprise multiple parallel gas-flow channels. A plurality offirst channels is open at the upstream side from which the exhaust gasstreams into the channels, and closed at the opposite end in flowdirection. The exhaust gas enters the first channels, passes the filterwalls and enters adjacent second channels, whereas the particles remaintrapped in the first channels. The second channels are closed at theupstream end and open at the opposite end downstream in flow direction,such that the exhaust gas exits the GPF.

Typically, the GPF comprises a catalytically active coating, typically aTWC coating. Thereby, the overall catalytic efficiency of the overallsystem can be enhanced and the performance can be increased. Typically,inner surfaces of the GPF, preferably all inner surfaces, are coatedwith a catalyst coating. Thus, inner walls of the filter channels or atleast portions thereof comprise a catalyst coating, such that theexhaust gas which passes the filter walls also flows through the porouscatalyst coating. Typically, the catalytic coating is located inside theporous filter walls, or onto the filter walls, or both, inside and ontothe filter walls. Thereby, the GPF can filter off particles, and at thesame time removes gaseous pollutants by catalytic chemical reaction. Thecatalyst may also support removal of particles, especially duringregeneration.

A “wash coat” (WC) is a thin, adherent coating of a catalytic or othermaterial applied to a carrier substrate. The carrier substrate can be ahoneycomb flow through monolith substrate or a filter substrate, whichis sufficiently porous to permit the passage of the gas stream beingtreated. A “wash coat layer” is defined as a coating that comprisessupport particles. A catalyzed wash coat comprises additional catalyticcomponents. The wash coats of the TWC1 and TWC2 of the system arecatalytic washcoats. Further, it is preferred that the GPF comprises acatalytic washcoat.

According to this application, the wash coat load is determined in g/l,wherein the weight in gram corresponds to all solids in the wash coat,whereas the volume is the total volume of the device, and not only thevoid volume of the device in the channels.

A “carrier” is a support, typically a monolithic substrate, examples ofwhich include, but are not limited to, honeycomb flow-through substratesfor the TWC and wall-flow filter substrates for the GPF. A “monolithicsubstrate” is a unitary structure that is homogeneous and continuous andhas not been formed by affixing separate substrate pieces together.Typically, the carrier is coated with a wash coat comprising thecatalyst.

An “OSM” refers to an oxygen storage material, which is an entity thathas multivalent oxidation states and can actively react with oxidantssuch as oxygen or nitric oxide (NO₂) under oxidative conditions, orreacts with reductants such as carbon monoxide (CO) or hydrogen underreduction conditions. Examples of suitable oxygen storage materialsinclude ceria or praseodymia. Delivery of an OSM to the wash coat layercan be achieved by the use of, for example, mixed oxides. For example,ceria can be delivered as an oxide of cerium and/or zirconium andmixtures thereof, and/or a mixed oxide of cerium, zirconium, and furtherdopants like rare earth elements, like Nd, Pr or Y.

As used herein, the “volume” of a device, such as a TWC or GPF, is thetotal volume of the device defined by its outer dimensions. Thus, thevolume is not only the void volume within the channels or within theporous structure of the device.

Preferably, the OSC is determined in fresh condition. The presence orabsence of oxygen storage capacity (OSC) can be determined by a jumptest. Thereby, the OSC in mg/L of a catalyst or system that is locatedbetween two A-sensors is calculated by the time offset of the two sensorsignals that is occurring after air-to-fuel ratio jumps (e.g. between λ0.95-1.05; see for example “Autoabgaskatalysatoren,Grundlagen—Herstellung—Entwicklung—Recycling—Ökologie”, ChristianHagelüken, 2^(nd) ed. 2005, page 62). The catalyst is in fresh conditionwhen it is put into use after manufacture.

Typically, the catalytic performance changes during operation and itsperformance may decrease. This phenomenon is known as aging. Thus, it ispreferred that the general catalyst performance is determined in agedcondition, as it is required by legislation.

According to the invention, the system comprises a TWC1, a GPF and aTWC2 in consecutive order. Such an arrangement of these three devicesconfers various advantages to the inventive system.

It is an advantage of the system that the GPF can be positionedrelatively close to the engine. Generally, GPFs require a relativelyhigh temperature for optimal performance and efficient regeneration.When the engine is started, the GPF is heated by the exhaust gas stream.When the GPF is located close to the engine, it is heated faster andachieves the high, optimal operation temperature earlier. The timewindow in which the filter is not operated efficiently is relativelysmall. In conventional systems, in which the GPF is a terminal deviceand/or located further away from the engine, more time is required forachieving the operation temperature for efficient three-way catalystactivity and soot oxidation.

Moreover, a GPF must be regenerated actively at defined time intervalsif it is located too far from the engine and therefore does not reachthe temperature that is required for soot burning. During regeneration,accumulated soot is burned at high temperature. If the requiredtemperature cannot be reached, the soot may not be burned completely andundesired side products, such as CO and hydrocarbons, can be formed inthe regeneration process. Therefore, it is advantageous for efficientregeneration that the GPF is located relatively close to the engine.

It is another advantage of the system that the partially purifiedexhaust gas, which is released from the GPF, can be subjected to furtherpurification by the downstream second TWC (TWC2). In known systems, theGPF is often the terminal purification device for final removal ofparticles from pre-purified exhaust gas. When the GPF is regenerated,the soot is oxidized and impurities, such as carbon monoxide (CO) orhydrocarbons (CH), can be formed. With a conventional system comprisinga terminal GPF, pollutants formed during regeneration are released intothe environment. According to the invention, the pre-purified exhaustgas from the GPF is subjected to downstream purification in the TWC2.Thereby, residual impurities which pass the GPF or which are formed inthe GPF can be removed or at least significantly reduced. The terminalTWC2 can ensure a final catalytic purification, which can function as afinishing step in the overall purification process.

It is a further advantage of the system that the TWC2 is located at aposition distant from the engine. It is a known problem in the technicalfield that catalytic devices, such as TWCs, undergo aging during use.Aging means that the activity and performance of the catalyst changesover lifetime, and usually tends to decrease. Generally, aging occursmore rapidly at high temperature. In the inventive system, the TWC2 ispositioned relatively far from the engine, which has the consequencethat less heat is transferred to the TWC2 than to the upstream devicesduring use. Therefore, the aging process of the TWC2 is comparably slowand the catalyst can maintain its efficiency and performance for aprolonged time. This can be advantageous for long-time use, especiallywhen emissions are monitored under RDE conditions. On the other hand,since the TWC2 is only the final catalyst for removing residualpollutants from the pre-purified exhaust gas, it can be acceptable thatits performance may not be optimal at certain time intervals due to itsposition relatively far from the engine. The terminal downstream TWC2can still efficiently clean up the relatively small amounts of residualpollutants, even at times when its temperature should not be as high asrequired for optimal function.

Overall, the system with the special arrangement of the TWC1, GPF andTWC2 allows highly efficient removal of gaseous and particulatepollutants from gasoline engine exhaust gas during standard use and fora prolonged time.

In a preferred embodiment, the platinum-group metal concentration (PGM)of the TWC2 is greater than the PGM of the GPF, wherein the PGM isdetermined in g/ft3 of the volume of the device. It can be advantageousto adjust the PGM of the TWC2 relatively high compared to the PGM of theGPF. The TWC2 is located at a position which is relatively far from theengine. During operation, the TWC2 will require more time to reach ahigh temperature compared to the TWC1 or GPF. Overall, the TWC2 will beoperated at lower temperatures on average, which means that thecatalytic efficiency may be lower at least at certain time intervals.However, the lower temperature also has the consequence that the TWC2will be less affected by aging and concomitant loss of catalyticefficiency. In the overall system, it can therefore be advantageous tocombine a TWC1 at a position close to the engine and the GPF, with theTWC2 at a position most remote from the gasoline engine, wherein theTWC2 is provided with a relatively high amount of catalyst. The TWC1 canprovide an initial high catalytic efficiency and performance. Theterminal TWC2 provides a final catalytic purification, which can beregarded as a finishing in the overall purification process. Overall, itcan be advantageous that the final purification with the TWC2 isrelatively efficient, stable and continuous, because the TWC2 is notsignificantly affected by aging and provided with a relatively highamount of catalyst.

In a preferred embodiment, the ratio of the platinum-group metalconcentration (PGM) of the TWC1 to the PGM of the TWC2 is from 1.1 to10, preferably from 1.25 to 9, more preferably from 1.45 to 5, whereinthe PGM is determined in g/ft3 of the volume of the device. Accordingly,the TWC1 can have an initial high catalytic efficiency and performance,whilst the terminal TWC2 provides a final catalytic purification, whichcan be regarded as a finishing in the overall purification process.Thereby, an advantageous balance of catalytic performances between theTWC1 and TWC2 can be achieved, which allows an efficient distributionand optimal use of expensive precious metals.

Further, a higher PGM of the TWC1 compared to the TWC2 can beadvantageous, because the diagnosis capability of the system can beimproved. Especially during on-board diagnosis, the catalyticperformance is commonly carried out by monitoring the first catalyticdevice in the system. When the catalytic performance of the TWC1 isrelatively high, on-board diagnosis can provide relatively goodapproximate results when monitoring only the TWC1. Thereby, a relativelygood correlation of the diagnosis result with real driving emissions ispossible.

In a preferred embodiment, the platinum-group metal concentration (PGM)of the TWC1 is at least 40% greater than the PGM of the TWC2, whereinthe PGM is determined in g/ft3 of the volume of the device. When the PGMof the TWC1 is significantly higher than of the TWC2, the advantagesdescribed above regarding overall catalytic efficiency, final finishing,cost efficacy and diagnosis capability can especially be obtained.

In a preferred embodiment, the platinum-group metal concentration (PGM)of the TWC1 is greater than the sum of the PGM of the GPF and TWC2,wherein the PGM is determined in g/ft3 of the volume of the device. Whenthe PGM of the TWC1 is adjusted accordingly, the overall system canespecially have a high catalytic efficiency under non-optimalconditions. Since the TWC1 is located closer to the engine than the GPFand TWC2, it can reach its optimal high operating temperature morerapidly and for longer time periods, and has a higher relative catalyticperformance. Therefore, it can be advantageous that a relatively highportion of the overall catalytic activity of the total system isconcentrated in the TWC1, whilst a significantly lower portion of thecatalytic activity is located in the GPF and TWC2. Overall, this can beacceptable, because the GPF is in contact with the pre-purified exhaustgas from the TWC1, which comprises significantly less gaseous pollutantsthan the original exhaust gas. Moreover, the TWC2 is only in contactwith the pre-purified exhaust gas from the GPF, which only comprisesresidual, relatively small amounts of gaseous pollutants. Overall, asystem can be provided in which the PGM is distributed between the threedevices, such that gaseous and particulate pollutants are efficientlyremoved whilst the pressure drop is kept low and engine performance ismaintained.

In a preferred embodiment, the total amount of platinum-group metal ofthe TWC1 is from 1 g to 15 g, preferably from 2 g to 10 g. In apreferred embodiment, the total amount of platinum-group metal of theGPF is from 0 g to 5 g, preferably from 0.05 g to 5 g, more preferablyfrom 1 g to 3 g.

In a preferred embodiment, the total amount of platinum-group metal ofthe TWC2 is from 0.1 g to 8 g, preferably from 0.2 g to 6 g. Overall, anefficient removal of pollutants can be achieved with the system when thetotal amount of platinum group metal is adjusted and distributedaccordingly.

In a preferred embodiment, the TWC1 comprises palladium and/or rhodium.In a preferred embodiment, the TWC2 comprises palladium and/or rhodium.Rhodium is especially efficient in removing NO_(x), whereas palladium isespecially efficient in removing CO. Therefore, the use of these metalsin these devices can be advantageous for efficient overall removal ofpollutants from the exhaust gas.

In a preferred embodiment, the percentage of rhodium of the total amountof platinum-group metal of the TWC2 is at least 10 wt. %, morepreferably at least 20 wt. %. This can be advantageous for efficientlyremoving NO_(x) by the TWC2.

In a preferred embodiment, the TWC2 does not comprise platinum. It canbe advantageous that the use of expensive platinum can be avoided in theTWC2, whilst an overall efficient removal of pollutants can be achieved.

In a preferred embodiment, the wash coat load (WCL) of the TWC2 isgreater than the WCL of the GPF, wherein the WCL is determined in g/l ofthe volume of the device.

Generally, a high WCL in the GPF can lead to a high pressure drop,because the exhaust gas has to pass the inner filter walls of the GPFand the catalytic wash coat on the filter walls. According to theinvention, it can be advantageous that the WCL of the GPF is relativelylow, because the pressure drop of the GPF, and therefore of the wholeexhaust gas purification system, can be kept low. In contrast, theexhaust gas usually does not traverse the filter walls of a three waycatalyst device. Thus, additional wash coat in the TWC2 will generallynot provide a comparable decrease of pressure drop as in the GPF.However, it is preferred that the GPF is provided with a wash coathaving catalytic activity. Overall, an efficient overall system can beprovided having high catalytic efficiency and low pressure drop.

Further, a significant WCL of the TWC2 can be advantageous, because itcan provide an efficient removal of all residual pollutants from thepre-purified exhaust gas not converted by or emitted from the GPF,especially when the GPF is regenerated. Thus, the overall system canprovide an efficient removal of pollutants.

Even further, the TWC2 is located relatively far from the gasolineengine. Thus, the TWC2 is subjected to and frequently operated at lowertemperatures than the other upstream devices. Consequently, the TWC2 isless affected by aging and concomitant deterioration of catalystperformance. Thus, since the catalytic efficiency of the TWC2 can berelatively stable, it can be advantageous to provide the TWC2 with arelatively high amount of wash coat.

In a preferred embodiment, the wash coat load (WCL) of the TWC2 is from100 g/l to 300 g/l, preferably from 150 g/l to 280 g/l, more preferablyfrom 175 g/l to 260 g/l. When the WCL of the TWC2 is adjustedaccordingly, a relatively good overall removal of pollutants with thecatalytic system is possible. Overall, the catalyst is used efficiently,because the TWC2 is located far away from the engine and less affectedby aging than the GPF or TWC1. Further, such amounts of wash coat aresuitable for providing an effective finishing of the pre-purifiedexhaust gas emitted from the GPF, whereby residual pollutants areremoved.

In a preferred embodiment, the wash coat load (WCL) of the GPF is from 0g/l to 150 g/l, preferably from 30 g/l to 130 g/l, more preferably from50 g/i to 110 g/l. When adapting the WCL of the GPF in such relativelylow ranges, the pressure drop can be adjusted to be low. Further, theoverall amount of catalyst in the system can be kept relatively low. TheGPF, which is coated with a relatively low amount of wash coat, canefficiently remove residual pollutants from the TWC1, which comprises arelatively high WCL.

In a preferred embodiment, the wash coat load (WCL) of the TWC1 is from150 g/l to 350 g/l, preferably from 180 g/l to 310 g/l, more preferablyfrom 200 g/l to 280 g/l. When the WCL of the TWC1 is used in suchrelatively high amounts, a good combination of high catalyticperformance of the TWC1 with efficient removal of residual pollutants bythe GPF and TWC2 is provided. It is also advantageous that the WCL ofthe TWC1 is adjusted within these relatively high ranges, because theTWC1 is located closest to the gasoline engine. Thus, it is heated morerapidly than the downstream devices and achieves the high optimaloperation temperature more often and for longer time periods. Further, ahigh WCL typically confers higher aging stability to the TWC1, which isespecially advantageous when the TWC1 is closed coupled to the engineand thus operated at higher temperatures Therefore, a relatively highwash coat load of the TWC1 can be advantageous for an effective initialand also total removal of pollutants. Overall, by adapting the WCL ofthe devices accordingly, an efficient use of total catalyst can beadjusted.

Further, a relatively high catalytic efficiency of the TWC1 can beadvantageous for diagnosis capability. Especially during on-boarddiagnosis, the catalytic performance is commonly carried out bymonitoring the first catalytic device in the system. When the wash coatload and catalytic performance of the TWC1 is relatively high, on-boarddiagnosis can provide relatively good results in approximation whenmonitoring only the TWC1. Thereby, a relatively good correlation of thediagnosis result with real driving emissions is possible.

In a preferred embodiment, the wash coat load (WCL) of the TWC1 isgreater than the WCL of the TWC2, wherein the WCL is determined in g/lof the volume of the device. This can be advantageous, because theupstream TWC1, which is located close to the gasoline engine, can beoperated more efficiently and more often at a high temperature andachieves a greater catalytic efficiency at high operation temperature.Since the TWC2 only removes residual pollutants, it is appropriate thatthe WCL is lower than of the TWC1. However, it can be advantageous thata downstream TWC2 is present, which removes pollutants from the TWC1 andGPF, and which is less affected by aging than the TWC1. In the overallsystem, it can be advantageous that on-board diagnosis can be performedwith the TWC1 and provides a reasonable correlation to total emissions.Overall, the TWC1 and TWC2 complement each other in the system andprovide, together with the GPF between them, an unexpected combinationof high filtration efficiency, high catalytic efficiency and lowpressure drop.

In one embodiment, the wash coat of the TWC1 is the same as for theTWC2. This means that the oxides comprised and concentrations thereofare the same in both wash coats. In another embodiment, the TWC1 andTWC2 comprise different wash coats.

TWC catalysts include oxygen storage materials (OSM) such as ceria thathave multi-valent states which allows oxygen to be held and releasedunder varying air to fuel ratios. Under rich conditions, when NO_(x) isbeing reduced, the OSM provides a small amount of oxygen to consumeunreacted CO and HC. Likewise, under lean conditions when CO and HC arebeing oxidized, the OSM reacts with excess oxygen and/or NO_(x). As aresult, even in the presence of an atmosphere that oscillates betweenfuel-rich and fuel-lean air to fuel ratios, there is conversion of HC,CO, and NO_(x) all at the same (or at essentially all the same) time.

According to the invention, the oxygen storage capacity (OSC) of the GPFis higher than the OSC of the TWC2, wherein the OSC is determined inmg/l of the volume of the device in fresh state. This can beadvantageous, because the GPF is located closer to the gasoline engine.Therefore, the GPF reaches the required high temperature for optimalperformance more rapidly and for longer time intervals than the TWC2.The relatively high OSC in the GPF can support and mediate therelatively efficient catalytic reaction in the GPF. In contrast, arelatively low OSC in the terminal TWC2, which is located further awayfrom the engine and does not achieve a high temperature as readily, cannonetheless be sufficient for removing the relative small amounts ofresidual pollutants, which are emitted from the GPF. Overall, thedistribution of the OSC in the system provides an efficient overallremoval of pollutants.

Further, a relatively high OSC of the GPF compared to the TWC2 can beadvantageous when the engine is operated alternately under richoperation conditions with lambda <1 and lean operation conditions withlambda >1 for short intervals of time. Such an operation mode is knownas wobbling or as wobble operation. In a wobbling mode, a high OSC canbe advantageous, because oxygen can be efficiently stored under leanoperation conditions and released under rich operation conditions.Accordingly, a higher OSC in the GPF supports an efficient overallremoval of pollutants under such conditions. In contrast, a high OSC ofthe TWC2 is less relevant for operation under wobble conditions, becausethe major portion of the pollutants was already removed in the TWC1 andGPF, such that the absolute concentrations of the residual pollutantswhich enter the TWC2 are comparably low.

In a preferred embodiment, the oxygen storage capacity (OSC) of the TWC1is higher than the OSC of the GPF, wherein the OSC is determined in mg/lof the volume of the device in fresh state. In a preferred embodiment,the OSC of the TWC1 is at least 40% greater than the OSC of the GPF. Ahigher OSC of the TWC1 can be advantageous, because it can support arelatively high catalytic efficiency of the TWC1. The TWC1 is locatedclosest to the gasoline engine and thus is operated more frequently atoptimal high temperature than the downstream GPF, and especially thanthe even more remote TWC2. Therefore, an efficient catalytic reactioncan occur more easily and more frequently in the TWC1 than in thedownstream devices. In the overall system, it can be generallyadvantageous if a relatively high catalytic turnover is mediated by theTWC1. Then, residual pollutants emitted from the TWC1 can be removed bythe downstream devices, which can have a lower OSC.

Moreover, a relatively high OSC of the TWC1 can be advantageous when theengine is operated alternately under rich operation conditions withlambda <1 and lean operation conditions with lambda >1 for shortintervals of time. Such an operation mode is known as wobbling or aswobble operation. In a wobbling mode, a high OSC can be advantageous,because oxygen can be efficiently stored under lean operation conditionsand released into the reaction under rich operation conditions.Accordingly, a high OSC in the TWC1 supports an efficient overallremoval of pollutants under such conditions. In contrast, a high OSC ofthe GPF is less relevant for operation under wobble conditions, becausethe major portion of the pollutants was already removed in the TWC1,such that the absolute concentrations of the pollutants are comparablylow in the GPF.

Even further, a high OSC in the TWC1 can be advantageous for efficientremoval of NO_(x) under lean operation conditions. A TWC1 having a highOSC can bind a large amount of oxygen under lean operation conditions.If the OSC would be too low, too much unbound oxygen can be presentunder lean conditions, and the reduction of NO_(x) to N₂ can beimpaired.

Further, a high OSC of the TWC1 can be advantageous for diagnosiscapability. Especially on-board diagnosis is commonly carried out withthe first catalytic device. When the upstream TWC1 has a high OSC, theresult of on-board diagnosis at the TWC1 will provide a reasonableindication of final emissions or real driving emissions (RDE).

In a preferred embodiment, the oxygen storage capacity (OSC) of the TWC1in fresh state is from 400 mg to 1250 mg, preferably from 500 mg to 900mg. This can be advantageous, because a high catalytic efficiency can beachieved at the TWC1 when the OSC is adjusted accordingly. As outlinedabove, this can be advantageous for overall catalytic efficiency,operation in the wobbling mode and diagnosis capability.

In a preferred embodiment, the ratio V_(cat)/V_(eng) is at least 1,wherein V_(cat) is the total catalyst volume of all devices and V_(eng)is the engine displacement of the gasoline engine. Thus, the totalcatalyst volume is at least the sum of the volumes of the TWC1, TWC2 andGPF. As used herein, the catalyst volume of a device is the overallvolume, and not only the internal void volume. A ratio of 1 or more isadvantageous, because a relatively high catalyst volume of all devicescan provide a high catalytic performance. The volume of the engine canapproximately be correlated to the amount of exhaust gas emitted duringoperation. If the total catalyst volume would be smaller than the enginevolume, the efficiency of the exhaust gas purification system can be toolow, especially under high mass flows that are observed under realdriving conditions. In order to achieve a high catalytic efficiency, arelatively high catalyst concentration may then have to be provided inthe catalytic devices, which could lead to an undesired increase of thepressure drop.

In a preferred embodiment, the ratio V_(cat)/V_(eng) is from 1 to 5,preferably from 1.1 to 4, more preferably from 1.2 to 3.5. If thecatalyst volume would be higher, the heat transfer from the gasolineengine to the catalytic devices could become insufficient. Generally, anefficient heat transfer from the gasoline engine to the downstreamcatalytic devices is required, such that the devices can attain theoptimal high operation temperature. Typically, such catalytic devicesare operated at a temperature of several hundred degrees Celsius for anoptimal performance and catalytic conversion. If the temperature isbelow the optimal temperature, the catalytic turnover can be decreased.Further, a compact integration of the catalytic system into a vehicle isdifficult, when the total catalytic volume is too high.

In a preferred embodiment, the volume of the TWC1 (V_(TWC1)) is from 20%to 50%, preferably from 30% to 40%, of the total catalyst volumeV_(cat). In this embodiment, it is preferred that the volume of the TWC1is larger than the volume of the TWC2. Overall, a relatively high volumeof the TWC1 can be advantageous, because the TWC1 can be provided with arelatively high catalytic efficiency. Accordingly, a major portion ofthe gaseous pollutants from the gasoline engine can be removed in theTWC1, which is located relatively close to the engine and attains arelatively high temperature more easily and frequently. This isespecially advantageous for efficient removal of pollutants underdynamic driving conditions, for example in urban traffic or after a coldstart of the engine. Further, diagnosis capability, especially foron-board diagnosis of catalytic efficiency, is usually carried out atthe upstream device, in this case the TWC1. Therefore, a high efficiencyof the TWC1 allows relatively good monitoring of the overall catalyticefficiency.

In a preferred embodiment, the volume of the GPF (V_(GPF)) is from 30%to 60%, preferably from 40% to 55%, of the total catalyst volumeV_(cat). In a specific embodiment, the total volume of the GPF is largerthan the total volume of the TWC1 and/or of the TWC2. A relatively highvolume of the GPF can be advantageous, because a relatively high volumecan be associated with a relatively low pressure drop. If the volume ofthe GPF would be too low, the pressure drop could increase, which couldyield to inefficient operation of the engine and increased carbondioxide emissions. A relatively high volume of the GPF can be especiallyadvantageous, if the GPF is a catalytic GPF, which is provided with acatalyst wash coat, preferably a TWC washcoat, which reduces the voidvolume in the device through which the exhaust gas can flow. Arelatively high volume of the GPF can also be advantageous for efficientstorage of particles and for efficient regeneration, when accumulatedsoot particles are removed under oxidizing conditions.

In a preferred embodiment, the volume of the TWC2 (V_(TWC2)) is from 10%to 40%, preferably from 15% to 35%, of the total catalyst volumeV_(cat). A lower volume of the TWC2, when compared to the TWC1 and/orGPF, can be advantageous, because removal of residual pollutants fromthe GPF may require less catalyst and thus catalyst volume in the TWC2.Therefore, an efficient overall system can be provided at relatively lowcosts and with favourable distribution of catalyst throughout the threedevices.

In a preferred embodiment, the ratio of the smallest diameter of the GPFto the length of the GPF is from 0.7 to 3, preferably from 0.75 to 1.6.When the dimensions of the GPF are adjusted accordingly, a relativelyefficient particle filtration and catalyst performance could be achievedwhile maintaining a relatively low backpressure.

In a highly preferred embodiment, the system comprises a turbochargerpositioned upstream from the TWC1. A turbocharger is a turbine-drivenforced induction device that increases an internal combustion engine'sefficiency and power output by forcing extra air into the combustionchamber. Therefore, a more efficient exhaust gas purification system canbe required if a turbocharger is present. The highly efficient inventivecatalytic system is especially suitable for purifying exhaust gas from agasoline engine and a turbocharger. Preferably, the turbocharger is theonly additional device between the gasoline engine and the TWC1.

Preferably, the distance from the outlet surface of the turbocharger tothe inlet surface of the TWC1 is from 1 cm to 40 cm, preferably from 2cm to 30 cm, more preferably from 2 cm to 20 cm. Preferably, thedistance is less than 10 cm or less than 5 cm. When the dimensions ofthe turbocharger and TWC1 are adapted accordingly, a close-coupledoperation of the TWC1 with the gasoline engine is possible. Then, heatcan be transferred more rapidly and efficiently from the engine to theTWC1 and downstream catalyst devices. This can be advantageous, becausethe catalytic reaction in the TWC1 and downstream devices is generallymore efficient at high temperature. Further, rapid heat transfersupports an efficient catalytic reaction after cold-start and underdynamic driving conditions, for example in urban traffic. Moreover, aclose-coupled system can be integrated directly in the space behind thegasoline engine. Accordingly, it is not necessary to integrate the TWC1,or downstream catalytic devices which are also close-coupled, into theunderbody of a vehicle. Thereby, a compact integrated catalyst systemcan be provided. Further, close-coupling of the TWC1 and the GPF to theengine generally can also provide a higher catalytic efficiency of theGPF and more efficient regeneration of the GPF.

In a preferred embodiment, the distance of the outlet surface of theTWC1 to the inlet surface of the GPF is from 1 cm to 60 cm, preferablyfrom 2 cm to 50 cm, more preferably from 3 cm to 40 cm. Preferably, thedistance is less than 20 cm or less than 10 cm. When keeping thedistance between the TWC1 and GPF relatively short, a close-coupledconnection of the GPF with the gasoline engine is possible. Thereby,heat can be transferred more efficiently and rapidly into the GPF, butalso the downstream TWC2. Further, the GPF can be integrated morecompactly into a vehicle. Further, the catalyst system can beregenerated more efficiently at high temperature and can have a highercatalytic turnover.

In a preferred embodiment, the distance of the outlet surface of the GPFto the inlet surface of the TWC2 is from 0 cm to 120 cm, preferably from1 cm to 110 cm, more preferably from 2 cm to 100 cm. Preferably, thedistance is less than 20 cm or less than 10 cm. Thereby, a close-coupledsystem is obtainable with additional advantages as described above.Overall, it is preferred that all devices of the system areclose-coupled to each other and to the gasoline engine.

In a preferred embodiment, the TWC1 comprises at least two differentwash coat layers. In a further preferred embodiment, the TWC2 comprisesone or two different wash coat layers. Preferably, the wash coat layersare laid over each other. In a preferred embodiment, different wash coatlayers can be located at different surfaces of the porous walls of theGPF. When combining different wash coat layers, catalytic coatings canbe combined which have different catalytic efficiency, resulting in anoverall system which is effective in removing different fractions ofpollutants.

In a further embodiment, the catalytic efficiency of the TWC1 is greaterthan that of the GPF with regard to removal of NO_(x), CO and/orhydrocarbons, when performance of the GPF is determined under the sameconditions as for the TWC1. This means that the performance of the GPFis determined without the upstream TWC1. This can be advantageous,because the TWC1 is closer to the engine and can be operated moreefficiently at a higher temperature. Further, a high wash coat load inthe TWC1 affects pressure drop less significantly than high wash coatload in the GPF, because the exhaust gas in the TWC1 does not have totraverse monolith filter walls.

In a further embodiment, the catalytic performance of the GPF is greaterthan that of the TWC2 with regard to removal of NO_(x), CO and/orhydrocarbons, when performance of the TWC2 is determined under the sameconditions as for the GPF. This means that the performances of the GPFand TWC2 are determined without further upstream exhaust gaspurification devices. This can be advantageous, because the levels ofgaseous pollutants in the GPF can be higher than in the pre-purifiedexhaust gas which enters the TWC2. Accordingly, relatively efficientremoval of pollutants in the GPF can be combined with final removal ofresidual pollutants in the TWC2. Overall, a system can be provided witheffective combination and adaptation of filtration efficiency, TWCefficiency and low pressure drop in the three devices, with a highlyefficient distribution of the catalytic material throughout the catalystsystem.

Preferably, the purified exhaust gas emitted from the TWC2 comprises thefollowing levels of pollutants (in mg/km):

CO: less than 1000, preferably less than 500, more preferably less than300

THC: less than 100, preferably less than 50, more preferably less than30

NO_(x): less than 60, preferably less than 40, more preferably less than30

PM: less than 0.005, preferably less than 0.002, more preferably lessthan 0.001

Preferably, the particle number (PN) is less than 6×10¹¹, preferablyless than 5×10¹¹. Preferably, these pollutant levels are determinedaccording to the standard tests defined in EURO6, test cycle WLTP (seeEU commission regulation 2007/715 and 2008/692 and regulations basedthereon 2017/1151, 2017/134).

Subject of the invention is also a method for purifying exhaust gasemitted from a gasoline engine, comprising the steps of:

-   -   (a) providing a gasoline engine and an exhaust gas purification        system of the invention, and    -   (b) passing exhaust gas emitted from the gasoline engine through        the system, such that the exhaust gas is purified by the system.

As outlined above, method uses the exhaust gas purification system asdescribed above, which is suitable for gasoline engines. It is adaptedto the specific exhaust gas and pollutants emitted from gasolineengines, which is different than exhaust gas from diesel engines.

Subject of the invention is also the use of the inventive exhaust gaspurification system for purifying exhaust gas from a gasoline engine.

The invention comprises the following embodiments:

-   1. An exhaust gas purification system for a gasoline engine,    comprising in consecutive order the following devices:    -   a first three-way-catalyst (TWC1), a gasoline particulate filter        (GPF) and a second three-way-catalyst (TWC2),    -   wherein the oxygen storage capacity (OSC) of the GPF is greater        than the OSC of the TWC2, wherein the OSC is determined in mg/l        of the volume of the device.-   2. The system according to embodiment 1, wherein the platinum-group    metal concentration (PGM) of the TWC2 is greater than the PGM of the    GPF, wherein the PGM is determined in g/ft3 of the volume of the    device.-   3. The system according to at least one of the preceding    embodiments, wherein the ratio of the platinum-group metal    concentration (PGM) of the TWC1 to the PGM of the TWC2 is from 1.1    to 10, preferably from 1.25 to 9, more preferably from 1.45 to 5,    wherein the PGM is determined in g/ft3 of the volume of the device.-   4. The system according to at least one of the preceding    embodiments, wherein the platinum-group metal concentration (PGM) of    the TWC1 is at least 40% greater than the PGM of the TWC2, wherein    the PGM is determined in g/ft3 of the volume of the device.-   5. The system according to at least one of the preceding    embodiments, wherein the platinum-group metal concentration (PGM) of    the TWC1 is greater than the sum of the PGM of the GPF and TWC2,    wherein the PGM is determined in g/ft3 of the volume of the device.-   6. The system according to at least one of the preceding    embodiments, wherein the total amount of platinum-group metal of the    TWC1 is from 1 g to 15 g.-   7. The system according to at least one of the preceding    embodiments, wherein the total amount of platinum-group metal of the    GPF is from 0 g to 5 g, preferably from 0.05 g to 5 g.-   8. The system according to at least one of the preceding    embodiments, wherein the total amount of platinum-group metal of the    TWC2 is from 0.1 g to 8 g.-   9. The system according to at least one of the preceding    embodiments, wherein the TWC1 comprises palladium and/or rhodium.-   10. The system according to at least one of the preceding    embodiments, wherein the TWC2 comprises palladium and/or rhodium.-   11. The system according to at least one of the preceding    embodiments, wherein the percentage of rhodium of the total amount    of platinum-group metal of the TWC2 is at least 10 wt. %.-   12. The system according to at least one of the preceding    embodiments, wherein the TWC2 does not comprise platinum.-   13. The system according to at least one of the preceding    embodiments, wherein the wash coat load (WCL) of the TWC2 is greater    than the WCL of the GPF, wherein the WCL is determined in g/l of the    volume of the device.-   14. The system according to at least one of the preceding    embodiments, wherein the wash coat load (WCL) of the TWC2 is from    100 g/l to 300 g/l, preferably from 150 g/l to 280 g/l, more    preferably from 175 g/l to 260 g/l.-   15. The system according to at least one of the preceding    embodiments, wherein the wash coat load (WCL) of the GPF is from 0    g/l to 150 g/l, preferably from 30 g/l to 130 g/l, more preferably    from 50 g/l to 110 g/l.-   16. The system according to at least one of the preceding    embodiments, wherein the wash coat load (WCL) of the TWC1 is from    150 g/l to 350 g/l, preferably from 180 g/l to 310 g/l, more    preferably from 200 g/l to 280 g/l.-   17. The system according to at least one of the preceding    embodiments, wherein the wash coat load (WCL) of the TWC1 is greater    than the WCL of the TWC2, wherein the WCL is determined in g/l of    the volume of the device.-   18. The system according to at least one of the preceding    embodiments, wherein the oxygen storage capacity (OSC) of the GPF is    greater than the OSC of the TWC2, wherein the OSC is determined in    mg/l of the volume of the device.-   19. The system according to at least one of the preceding    embodiments, wherein the oxygen storage capacity (OSC) of the TWC1    is greater than the OSC of the GPF, wherein the OSC is determined in    mg/l of the volume of the device.-   20. The system according to at least one of the preceding    embodiments, wherein the oxygen storage capacity (OSC) of the TWC1    is from 400 mg to 1250 mg, preferably from 500 mg to 900 mg.-   21. The system according to at least one of the preceding    embodiments, wherein the ratio V_(cat)/V_(eng) is at least 1,    wherein V_(cat) is the total catalyst volume of all devices and    V_(eng) is the engine displacement of the gasoline engine.-   22. The system according to at least one of the preceding    embodiments, wherein the ratio V_(cat)/V_(eng) is from 1 to 5,    preferably from 1.1 to 4, more preferably from 1.2 to 3.5.-   23. The system according to at least one of the preceding    embodiments, wherein the volume of the TWC1 (V_(TWC1)) is from 20%    to 50% of the total catalyst volume V_(cat), preferably from 30% to    40%.-   24. The system according to at least one of the preceding    embodiments, wherein the volume of the GPF (V_(GPF)) is from 30% to    60% of the total catalyst volume V_(cat), preferably from 40% to    55%.-   25. The system according to at least one of the preceding    embodiments, wherein the volume of the TWC2 (V_(TWC2)) is from 10%    to 40% of the total catalyst volume V_(cat), preferably from 15% to    35%.-   26. The system according to at least one of the preceding    embodiments, wherein the ratio of the smallest diameter of the GPF    to the length of the GPF is from 0.7 to 3, preferably from 0.75 to    1.6.-   27. The system according to at least one of the preceding    embodiments, wherein the system comprising a turbocharger positioned    upstream of the TWC1, wherein the distance of the outlet surface of    the turbocharger to the inlet surface of the TWC1 is from 1 cm to 40    cm, preferably from 2 cm to 30 cm, more preferably from 2 cm to 20    cm.-   28. The system according to at least one of the preceding    embodiments, wherein the distance of the outlet surface of the TWC1    to the inlet surface of the GPF is from 1 cm to 60 cm, preferably    from 2 cm to 50 cm, more preferably from 3 cm to 40 cm.-   29. The system according to at least one of the preceding    embodiments, wherein the distance of the outlet surface of the GPF    to the inlet surface of the TWC2 is from 0 cm to 120 cm, preferably    from 1 cm to 110 cm, more preferably from 2 cm to 100 cm.-   30. The system according to at least one of the preceding    embodiments, wherein the TWC1 comprises at least two different wash    coat layers.-   31. The system according to at least one of the preceding    embodiments, wherein the TWC2 comprises one or two different wash    coat layers.-   32. The system according to at least one of the preceding    embodiments, wherein the catalytic performance of the TWC1 is    greater than that of the GPF with regard to removal of NO_(x), CO    and/or hydrocarbons, when performance of the GPF is determined under    the same conditions as for the TWC1.-   33. The system according to at least one of the preceding    embodiments, wherein the catalytic performance of the GPF is greater    than that of the TWC2 with regard to removal of NO_(x), CO and/or    hydrocarbons, when performance of the TWC2 is determined under the    same conditions as for the GPF.-   34. A method for purifying exhaust gas emitted from a gasoline    engine, comprising the steps of:    -   (a) providing a gasoline engine and an exhaust gas purification        system of any of the preceding embodiments, and    -   (b) passing exhaust gas emitted from the gasoline engine through        the system, such that the exhaust gas is purified in the system.-   35. Use of an exhaust gas purification system of any of the    preceding embodiments for purifying exhaust gas from a gasoline    engine.

The invention claimed is:
 1. An exhaust gas purification system for agasoline engine, comprising in consecutive order the following devices:a first three-way-catalyst (TWC1), a gasoline particulate filter (GPF)and a second three-way-catalyst (TWC2), wherein the oxygen storagecapacity (OSC) of the GPF is greater than the OSC of the TWC2, whereinthe OSC is determined in mg/l of the volume of the device, and whereinthe platinum-group metal concentration (PGM) of the TWC1 is greater thanthe sum of the PGM of the GPF and TWC2, wherein the PGM is determined ing/ft3 of the volume of the device.
 2. The system according to claim 1,wherein each of the TWC1, GPF and TWC2 is a separate unit.
 3. The systemaccording to claim 2, wherein each of the different units are providedin separate housings.
 4. The system according to claim 1, wherein theplatinum-group metal concentration (PGM) of the TWC2 is greater than thePGM of the GPF, wherein the PGM is determined in g/ft3 of the volume ofthe device.
 5. The system according to claim 1, wherein the ratio of theplatinum-group metal concentration (PGM) of the TWC1 to the PGM of theTWC2 is from 1.1 to 10, wherein the PGM is determined in g/ft3 of thevolume of the device, and/or wherein the platinum-group metalconcentration (PGM) of the TWC1 is at least 40% greater than the PGM ofthe TWC2, wherein the PGM is determined in g/ft3 of the volume of thedevice.
 6. The system according to claim 5, wherein the ratio of theplatinum-group metal concentration (PGM) of the TWC1 to the PGM of theTWC2 is from 1.45 to
 5. 7. The system according to claim 1, wherein thetotal amount of platinum-group metal of the TWC1 is from 1 g to 15 g,and/or wherein the total amount of platinum-group metal of the GPF isfrom 0 g to 5 g.
 8. The system according to claim 1, wherein the totalamount of platinum-group metal of the TWC2 is from 0.1 g to 8 g.
 9. Thesystem according to claim 1, wherein the TWC1 comprises palladium and/orrhodium, and/or wherein the TWC2 comprises palladium and/or rhodium. 10.The system according to claim 1, wherein the percentage of rhodium ofthe total amount of platinum-group metal of the TWC2 is at least 10 wt.%, and/or wherein the TWC2 does not comprise platinum.
 11. The systemaccording to claim 2, wherein the wash coat load (WCL) of the TWC2 isgreater than the WCL of the GPF, wherein the WCL is determined in g/l ofthe volume of the device.
 12. The system according to claim 1, whereinthe wash coat load (WCL) of the TWC2 is from 100 g/l to 300 g/1.
 13. Thesystem according to claim 12, wherein the WCL of the TWC2 is from 175g/l to 260 g/l.
 14. The system according to claim 2, wherein the washcoat load (WCL) of the GPF is from 0 g/l to 150 g/l.
 15. The systemaccording to claim 14, wherein the WCL of the GPF is from 50 g/l to 110g/l.
 16. The system according to claim 1, wherein the wash coat load(WCL) of the TWC1 is from 150 g/l to 350 g/1.
 17. The system accordingto claim 1, wherein the wash coat load (WCL) of the TWC1 is greater thanthe WCL of the TWC2, wherein the WCL is determined in g/l of the volumeof the device.
 18. The system according to claim 1, wherein the oxygenstorage capacity (OSC) of the TWC1 is greater than the OSC of the GPF,wherein the OSC is determined in mg/l of the volume of the device. 19.The system according to claim 1, wherein the oxygen storage capacity(OSC) of the TWC1 is from 400 mg to 1250 mg.
 20. The system according toclaim 19 wherein the OSC of the TWC1 is from 500 mg to 900 mg.
 21. Thesystem according to claim 1, wherein the distance of an outlet surfaceof the TWC1 to an inlet surface of the GPF is from 1 cm to 60 cm, andthe distance of an outlet surface of the GPF to an inlet surface of theTWC2 is from 1 cm to 110 cm.
 22. The system according to claim 1,wherein the GPF comprises porous filter walls and a catalytically activecoating that coats the inside of the porous filter walls.